The invention concerns devices for controlling the pitch of rotor aircraft rotor blades, particularly for a helicopter main rotor, the rotor being of the type for which each blade is, on the one hand, rotated around an axis of rotation of a rotor shaft, or rotor axis, by means of a hub rotating integrally with the shaft, and, on the other hand, rotating integrally, around a longitudinal blade pitch change axis, with at least one pitch lever controlled by a corresponding pitch rod.
The invention relates more exactly to a pitch control device of the type including a swash-plate assembly, and such that each pitch rod is connected to a plate rotating with the rotor and belonging to the swash-plate assembly, in which the rotating plate is rotary mounted on a non-rotating plate, restrained against any rotation around the rotor axis, the two plates being annular, surrounding the rotor axis and being oscillating mounted by the non-rotating plate on a central ball joint, centered on the rotor axis and mounted sliding parallel to the rotor axis around a cylindrical guide coaxial to the rotor axis and non-rotating around said rotor axis, with the result that the swash-plates can be translated axially, i.e. parallel to the rotor axis, and tilted in any direction around the rotor axis, being activated by control actuators connecting the non-rotating plate to the structure of the rotor aircraft, for the control respectively of the collective pitch and the cyclic pitch of the blades.
Generally, the two plates surround the rotor shaft and the control actuators are three servo controlled jacks or similar actuators placed between the swash-plates and the rotor aircraft structure, and articulated by ball joints at their lower and upper ends respectively on this structure and in devises of the non-rotating plate, whereas the pitch rods extend between the swash plates and the pitch levers of the rotor blades, and are articulated by ball joints at their upper and lower ends respectively on the pitch levers and in devises distributed at the periphery of the rotating plate.
Most often, the cylindrical guide is fixed in relation to the rotor aircraft structure and where the swash-plates surround the rotor shaft, the cylindrical guide is tubular, surrounds the rotor shaft, and is fixed to a housing integral with the structure of the rotor aircraft and surrounding the connection between the rotor shaft base and the main gear box.
Most often also, the rotating and upper or non-rotating and lower connection part or parts which connect respectively the rotor to the rotating plate to rotate this latter, and the structure to the non-rotating plate, to restrain the latter, is or are one or more torque links with two scissor-articulated arms and connected to each other by a pivot or a hinge, which enables the two arms of each torque link to move apart or draw together in order to enable movements in the direction of the rotor axis, since the upper and lower arms of a rotating torque link are moreover articulated on the shaft or the hub of the rotor and on the rotating plate respectively, and as the lower and upper arms of a non-rotating torque link are moreover articulated on the structure of the rotor aircraft and on the non-rotating plate.
The drawbacks of these rotating and non-rotating torque links are that their articulations comprise traditional, unreliable, bearings, or, on more recent helicopters, self-lubricating ball joints and bearings including numerous carbide pins and rings, which are heavy and expensive.
Furthermore, a certain overall height or axial requirement (parallel to the rotor axis) and width requirement is necessary for the torque links to operate correctly. So, to facilitate their loading for example in the hold of a transport airplane and to improve their capacity to be accommodated in a hangar, for example of a ship, modern helicopters must offer the best possible vertical or axial compactness of the shaft and the main rotor assembly and of the swash-plate assembly which surrounds it. But reduction in the height or axial size of such an assembly is limited by the interference between the two arms of a torque link during folding of the latter and by the maximum swiveling angles permitted by the construction of the articulation ball joints of the torque links.